Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) technology is a mobile broadband wireless communication technology in which transmissions from base stations (referred to as enhanced or evolved Node Bs (eNBs)) to mobile stations (referred to as User Equipment devices (UEs)) are sent using Orthogonal Frequency Division Multiplexing (OFDM). OFDM splits the signal into multiple parallel sub-carriers in frequency. The basic unit of transmission in LTE is a Resource Block (RB), which in its most common configuration consists of 12 subcarriers and 7 OFDM symbols (one slot). A unit of one subcarrier and 1 OFDM symbol is referred to as a Resource Element (RE). Thus, an RB consists of 84 REs. In this regard, FIG. 1 is a schematic diagram showing LTE physical resources.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds (ms), each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms, as illustrated in FIG. 2. FIG. 3 is a schematic diagram of a downlink subframe. An LTE radio subframe is composed of multiple RBs in frequency with the number of RBs determining the bandwidth of the system and two slots in time. The two RBs in a subframe that are adjacent in time are denoted as an RB pair.
The signal transmitted by the eNB in a downlink subframe (downlink being the link carrying transmissions from the eNB to the UE) may be transmitted from multiple antennas, and the signal may be received at a UE that has multiple antennas. The radio channel distorts the transmitted signals from the multiple antenna ports. In order to demodulate any transmissions on the downlink, a UE relies on Reference Signals (RSs) that are transmitted on the downlink. These RSs and their position in the time-frequency grid are known to the UE and hence can be used to determine channel estimates by measuring the effect of the radio channel on these symbols.